1. Field of the Invention
The present invention relates to a zoom lens, and more particularly, to a compact, high magnification zoom lens having a long back focal length and telecentricity.
2. Description of the Related Art
Generally, a zoom lens used in a still camera and a video camera requires excellent optical performance, high magnification, and compactness. Moreover, with the widespread use of electronic equipment such as personal digital assistants (PDAs) and other mobile terminals, digital cameras and digital video units are increasingly being installed in such equipment. As a result, there is a growing demand for increasingly compact cameras and camera lenses.
Japanese Patent Publication No. 2002-156581 discloses a zoom lens. As shown in FIGS. 1A and 1B, the zoom lens includes a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third, a fourth, and a fifth lens group having positive refractive powers, and an aperture diaphragm in the vicinity of the third lens group. The second lens group is linearly moved toward the third group. The fourth lens group is moved from the vicinity of the fifth lens group to a long focus end position located in the vicinity of the third lens group. The second and fourth lens groups function in association to accomplish magnification change.
In such a zoom lens, since the second and fourth lens groups are moved to change magnification and the first and third lens group are fixed, the entire optical system becomes too large to compensate for performance at a wide angle end.
Japanese Patent Publication No. 2002-365548 also discloses a zoom lens. As shown in FIGS. 2A through 2C, the zoom lens includes sequentially from an object side a first lens group G1 having a positive refractive power, a second lens group G2 having a negative refractive power, and a third lens group G3, a fourth lens group G4, and a fifth lens group G5 each having positive refractive powers. The first through fifth lens groups G1, G2, G3, G4, and G5 are moved to change magnification. This zoom lens includes many lenses and is thus suitable for high magnification but is not appropriate for miniaturization. In addition, since this zoom lens uses a charged-coupled device, a quartz crystal filter is required to prevent a moire phenomenon caused by a periodic structure of the charged-coupled device. In consideration of the thickness and position of the quartz crystal filter, a long back focal length must be secured. Particularly, telecentricity of light incident onto an image plane is very important. However, in such a structure, it is difficult to secure the long back focal length since the fifth lens group G5 is moved to accomplish magnification change, and it is difficult to accomplish the telecentricity.